1. Field of the Invention
The present invention relates generally to improvements in sensing devices and, more particularly, but not by way of limitation, to devices used to detect the passage of a travelling liquid displacer in a pipe type meter prover.
2. Brief Description of the Prior Art
In commerce and industry, a need often exists to measure the amount of liquid that is passed through a conduit and appropriate meters, that can be mounted on conduits, have been developed for this purpose. In some circumstances, the accuracy of a meter on a conduit can be an important consideration to parties having an interest in the passage of the liquid through the conduit and the oil industry provides an example of such circumstances. Title to oil may pass from one party to another party with the passage of the oil through a pipeline and the price the buyer pays the seller is determined by the amount of oil he has received as determined by a meter on the pipeline. Since even a small inaccuracy in the meter, over an extended period of time, can make a substantial difference in the amount of money that changes hands, meter accuracy is important to both the buyer and the seller of the oil. It is common practice in these circumstances for the contract of sale of the oil to call for the periodic proving, or calibration, of the meter and a prover is used for this purpose.
A prover is essentially a pipe that includes a traveling liquid displacing seal member, often an inflated elastic sphere or piston, and a pair of detectors to provide signals that indicate the passage of the displacer at two spaced locations on the prover at which the detectors are located. The prover is constructed to be connected, via appropriate valves, to a conduit upon which the meter to be proved is mounted such that liquid passing through the meter is diverted from the conduit to also pass through the prover and move the displacer between the two detectors. The detectors, by marking the location of the displacer at two different points in the prover, define a volume of liquid which has passed through the meter and such volume is used to calibrate the meter. In general, two types of pipe provers are used: the unidirectional type in which the displacer is passed from one end of the prover to the other and then returned to the starting end for a subsequent run through the prover and the bidirectional type in which the displacer is passed back and forth through the prover during each proving run.
In order for the prover to carry out its purpose of proving meters, the prover itself must be calibrated so that the amount of liquid that passes through the prover, between detections of the displacer, will be known. Such calibration is carried out by the water draw method in which water is passed through the prover with the effluent from the prover, during the passage of the displacer between the detectors, being captured for measurement using containers whose volumes have been very accurately determined. This calibration of the prover is a time consuming and difficult task, and almost entirely reliant upon the accuracy and repeatability of the detectors, with the result that such calibration is not lightly undertaken.
Initially, it will be noted that almost all prior art prover detectors include a piston that has a portion extending into the prover to be engaged by the displacer and forced outwardly of the prover as the displacer passes the detector. In one type of prior art detector, a portion of the piston passes through a dynamic seal so that a mechanical coupling to an appropriate switch can provide the appropriate closure of electrical contacts in the switch. In another type of prior art detector, the piston carries a magnet, or several magnets, and the detector includes a reed switch that can be actuated by the magnet, or magnets, as the piston is moved by the travelling displacer member to bring the magnets near the reed switch. Both of these types of detectors give rise to inaccuracies in the relative positions of the displacer and the detector that are related to the position of the piston for which closure of electrical contacts occurs.
In the mechanical type detector, the use of a dynamic seal can have the effect of making the closure point for the contacts dependent upon pressure in the prover. At higher pressures, the dynamic seals become stiff so that a greater force must be exerted on the piston to cause the piston to move outwardly of the prover. In addition, springs in the detector that return the piston to a position to engage the displacer must have a relatively high spring constant to accomplish this purpose. As a result, the displacer, which as noted above is often an inflated elastic sphere, is called upon to exert a considerable force on the piston and the displacer can be distorted thereby. Where this occurs, the position of the displacer relative to the detector, for closure of contacts in the detector during a proving run, can vary from the position of the displacer relative to the detector that occurred during the calibration of the prover. That is, the calibrated volume of liquid that passes through the prover as the displacer passes between the detectors, as determined by the calibration of the prover, does not apply in circumstances under which the prover is used to prove a meter so that the proving of the meter will be inaccurate. This problem is especially severe where the prover is a unidirectional prover; in a bidirectional prover, inaccuracies stemming from the position of the displacer member for closure of the detector contacts tend to be offsetting. Even so, however, it is not desirable that the inaccuracy exist even for the bidirectional prover since the degree to which the position of the displacer is inaccurate, and the effect of such inaccuracy on the calibration of the meter, cannot be known.
The magnet and reed type detector can simlarly give rise to faulty detection of a precise location of the displacer relative to the detector at which contacts in the detector close because of inherent asymmetries that exist in such detectors. In particular, should the piston in the detector rotate about the axis of movement of the piston from the prover, the geometry of the magnetic field produced by the magnet, or magnets, carried by the piston will change at the reed switch so that the point at which the reed switch closes becomes an undetermined function of the orientation of the piston about its axis of movement. Again, the result is that the position of the displacer for closure of contacts in the detector cannot be relied upon so that the calibration of the prover cannot be relied upon.
Other problems, stemming from this basic lack of reliability of prior art detectors, are also encountered. In order to minimize the above problems, the detectors are tuned during calibration of a prover and such tuning cannot be duplicated in the factory. For example, in the case of the magnet and reed type detector, such tuning might be carried out to find a position of the reed for which the closure point of the piston is a slowly varying function of the relative orientations of the reed and magnets in the detector.